1. Field of the Invention
The present invention relates to a radar device that is mounted on a vehicle for detecting an object (target) such as a preceding vehicle that exists around an own vehicle and measuring a distance thereof from the own vehicle and a relative velocity of the object with respect to the own vehicle.
2. Description of the Related Art
A radar device installed on an own vehicle transmits an electromagnetic wave as a transmitter signal, receives a reflected signal that is reflected by a target such as a preceding vehicle as a receiver signal, and calculates a distance and a relative velocity between the own vehicle and the target on the basis of a frequency analysis result with respect to a difference between the transmitter signal and the receiver signal. The radar device is used in an obstacle proximity warning system, an inter-vehicle distance control system, or a traffic jam following system.
In the radar device, when a receiver signal that has been reflected by the target and an electromagnetic wave (interference signal) that has been transmitted from another on-vehicle radar or a communication device are received at the same time, there occurs a so-called interference in which a signal intensity in the frequency analysis result largely increases, and a signal intensity for measuring the distance to the target and the relative velocity of the target is buried. In this situation, although there exists the target, it is impossible to measure the distance to the target and the relative velocity of the target, which causes a problem in controlling the vehicle.
Under the above-mentioned circumstances, when the interference occurs, it is necessary to surely detect the interference, and notify or warn a vehicle system side of the deterioration in the performance of the radar device. As the interference detecting method of this type, there is a method of temporarily stopping transmittance of the electromagnetic waves, receiving only the electromagnetic waves (interference signal) which are transmitted from another on-vehicle radar or the communication device, and detecting the presence or absence of the interference signal based on the frequency analysis results of the receiver signal (see, for example, JP 2004-163340 A).
However, in the radar device which is disclosed in JP 2004-163340 A, because the transmission of the electromagnetic waves stops during the interference detecting operation, the measurement of the distance to the target and the relative velocity of the target must be suspended, which is not efficient. Also, when the interference detecting operation is conducted with high frequency in order to enhance the sensitivity of the interference detection, there arises such a problem that a period of time during which the distance to the target and the relative velocity of the target can be measured is reduced.
Also, in the case of scanning with an antenna, there is a method of conducting the interference detecting operation in a specific beam direction, for example, by only an end beam of the antenna scanning.
However, influences of the interference on the respective beams are different depending on the arrival directions of the interference signal. For that reason, there is a case where the interference signal cannot be detected when the interference detecting operation is conducted in only a specific direction.
Also, there is proposed a method of detecting the interference signal based on whether the signal intensity increases or not, since the signal intensity in the frequency analysis results which are obtained by a difference between the transmitter signal and the receiver signal increases when the electromagnetic waves (interference signals) which are transmitted from another radar device or a communication device are received.
However, because there is no distinction between a case where the signal intensity increases due to the receiver signal that is reflected by the target and a case where the signal intensity increases due to the interference signal, the above-mentioned method is not suitable for detecting the interference signal.